The present invention concerns systems and methods for braking of and traction for wheeled vehicles, and more particularly optimum braking and traction under any road or ground conditions.
The principles utilized in the present invention are to a certain extent described in Applicant's previous Norwegian patent application no. 88.1513, which hereby is incorporated by reference.
Traction is in the present context meant to be understood as the powered acceleration of a wheel, i.e. the opposite of braking.
The present invention has particularly been developed in connection with the need of ability to undertake braking, or possibly traction, while utilizing maximally the attainable friction coefficient between a wheel and a ground course, particularly with pneumatic tires and the surface of runways and roads. Said friction coefficient, usually designated .mu., depends on weather conditions, and may therefore vary considerably. As will be explained in more detail later in this specification, the friction coefficient .mu. also is strongly dependent on the so-called "slip" conditions of the wheel. A precise and fast adjustment to the optimum braking force value is of considerable benefit for example to a pilot in an airplane at departure and landing, because such an adjustment renders possible optimum braking and avoidance of skidding.
For a railway train also the acceleration phase is important, since optimum utilization of power and minimum wear of material is achieved by rapid adjustment of the optimum traction, so that unnecessary and damaging wheel skidding is avoided and the train reaches its cruising speed in the fastest possible manner.
Also for ordinary motor vehicles, i.e. cars, lorries, tractors etc., good automatic braking systems, i.e. non-blocking systems, are of interest for safety reasons. In particular cases, also an effective traction control may be advantageous, for example in sports-like driving and in driving outside roads in rugged terrain.
Braking systems have been developed previously, which systems seek to avoid blocking the wheels during a hard braking action, but it has turned out that these systems may give results which are not completely satisfactory, which fact has led to a few airplane accidents in the braking phase, which accidents might have been avoided. The problem is often that the existing systems do not really know the state of movement of the airplane, only whether the wheels are rotating or not.
Previously known braking systems for cars are also usually based upon sensing the rotational state of the wheels--if the wheel stops rotating, the braking power is cut. In some related systems, i.e. mainly systems for measuring friction, particularly between an airplane wheel and the runway, the friction force is measured for a wheel during braking, at a certain slip factor for the wheel. The slip factor gives an expression of the slip or the sliding between a rotating wheel and the ground. It is necessary that a wheel is subjected to slip in order that horizontal forces be transferred when the wheel is rolling. In this connection the slip factor S is defined as ##EQU1## in which n.sub.k is the number of revolutions of a freely rotating wheel in contact with the ground, and n.sub.b is the number of revolutions of the braked wheel.
In the formula above the slip factor is a number between 0 and 1, however said factor may also be expressed in percent, i.e. ##EQU2## and the slip factor will then be a number between 0 and 100. 100% slip thus means a locked, braked wheel (total panic braking), and 0% means no braking of the same wheel, i.e. a freely rotating wheel.
In airport runways it has become usual to make friction measurements with a slip factor between 15 and 17%. However, the friction coefficients or friction forces obtained in these measurements, will only be correct, i.e. the maximum possible values, for one particular type of ground conditions. It has turned out as a fact that the slip factor which provides maximum runway friction, will be lower under dry summer conditions and higher under slippery winter conditions. In other words, a higher slip factor should be set for use under slippery winter conditions than under dry summer conditions.
Non-blocking braking systems comprise means for improving the braking action for a wheeled vehicle by providing a reduction of the braking force acting on a wheel if said wheel tends to lock or block in a manner which will give starting of skidding after the actual brake application, and said means will thereafter provide a new increase in the braking force without necessitating any change in the current braking maneuver (of a car driver or operator using the brake) which has caused the actual brake application. Such braking systems are advantageous as to reducing the danger of swerving with blocked wheels, and for maintaining the steering ability during braking, and they may also provide a reduction of the braking distances.
When a braking force is supplied to a wheel for reducing the vehicle speed, a certain percentage of slip is introduced, i.e. the braked wheel rotates slower than the free rolling speed which the wheel would have for maintaining the instantaneous vehicle speed, and this fact is due to the friction force between the wheel tire and the ground. When the braking force is increased, there is also an increase in said friction force, and simultaneously the percentage of slip increases until the friction force reaches a maximum value at a percentage of slip (slip factor) usually between 10 and 30%, and thereafter the friction force decreases with a further increase in the braking force, and at the same time the slip factor increases to 100%, with locking of the braked wheel. Still a friction force is present to retard the vehicle (sliding friction), however this value is lower, and often essentially lower than the maximum possible friction force.
The well known expression for the friction coefficient .mu. is given by EQU .mu.=F/N
in which N is the normal force from the ground on the object lying on the ground, i.e. in this case the wheel, and F is the friction force. If one at first supposes that the normal force N, which ordinarily is equal to or directly proportional to the weight of the object (in the case of a car, the car weight divided by the number of wheels), is maintained constant, it is clear that the friction coefficient .mu. is directly proportional to the friction force F. What has been stated above regarding variation of the friction force, can therefore in this case equally well be stated regarding the friction coefficient .mu.. Thus, .mu. varies in accordance with the value of the slip factor (see FIG. 1).
From Swedish laid-open publication 394 984 a non-blocking braking system is known, in which the braking force is controlled in such a manner that the wheel supposedly is maintained rolling within the region of maximum friction force between the tire and the road, i.e. within a region which possibly may be more specifically defined and in which region optimum slip percentage is achieved, but the necessary information about the actual slip percentage is not provided in the method according to said laid-open publication, paradoxically, nor can any information be found regarding the forces in question. An electrical control circuit is used for detecting wheel rotational states where wheel blocking threatens as a consequence of too powerful braking, together with an electromagnet valve which is activated via the control circuit for reducing the brake fluid pressure. More specifically, this previously known solution is based upon utilization of means for providing a DC signal, the amplitude of which is a function of the rotation speed of the wheel, and no other physical parameters than wheel rotation speed are detected. Thus, the system does not know how the vehicle moves, and in reality the system functions poorly just in those cases where it is most needed, namely when braking the vehicle under slippery conditions, i.e. with typically low values of the friction coefficient. A problem will also exist at low speeds, since the measuring signal in this case will not be very useful. The problem is, as previously mentioned, that irrespective of electronic "smartness" as to use of reference values for maximum allowed retardation of wheel rotation, the basic information about how the vehicle is really moving, is missing.
From Swedish laid-open publication 413 082 there is known a slide-preventing control device for braking a vehicle. Also in this case the aim is preferentially to maintain the slip within a region which is coordinated with a maximum friction coefficient. Periodic modulation of the wheel torque is used together with wheel acceleration measurements in order to determine the direction of change of the friction coefficient from an optimum value as a function of slip, while an integral/proportional control of the pulse modulation is supposed to render possible a compensated variation of the wheel torque and the slip state into a state providing optimum friction force. The control device may be utilized for preventing skidding of the vehicle not merely during braking, but also during acceleration. However, this system has the same weaknesses as the previously mentioned system, since merely the wheel rotation and the variation thereof is detected.
In contrast hereto, there is known from Swedish laid-open publication 382 781 a system for preventing blocking of a vehicle wheel, which system lies closer to the system dealt with in the present application. In SE 382 781 one measures the horizontally acting reaction force on the wheel axle, which reaction force is proportional to the friction force between the wheel and the ground, and a signal representing the measured road parallel force, is used for controlling the braking power in such a direction that the friction force is maximized. This system can only be utilized in connection with braking, and the system does not take into account variations in the road normal force N. It should at this point be emphasized that particularly during braking or acceleration phases the reaction force will cause a fast change of the road normal force for the individual wheels. When N varies, F, that is the road parallel force, may experience a necessary change due to the change in N. The known system will misinterpret such a situation, and believe that the change in F is due to applying an incorrect braking power, even though the instantaneous braking power is actually correct. Possibly the direction of the immediately necessary change of braking power may be misinterpreted when F is influenced by a change in N. Thus, the known system will be "fooled" in a situation like this, and will therefore use more time for adjusting to the correct slip value than what is optimum.